The present invention relates to solid state shear pulverization of multi-component polymeric blends, including thermodynamically incompatible polymers, to form without compatibilizing agents pulverized particulates that are directly melt processable as powder feedstock to shaped articles of manufacture by conventional blow molding, rotational molding, extrusion, and spray coating techniques without color streaking in the resulting articles of manufacture. Importantly, polymer blends formed of unsorted, post-consumer and post-industrial plastic film waste can be formed by solid state shear pulverization into polymeric particulates having surprisingly high notched izod impact strength.
Decreasing landfill space and rapidly rising disposal costs have forced many municipalities to begin curbside recycling of post-consumer plastic (polymeric) waste.
In 1997, municipal solid waste (MSW) generation in the U.S. totaled 217 million tons; plastics constituted 21.5 million tons, or 9.9 percent by weight of the total MSW generated, of which only 1.1 million tons have been recovered, (5.2% of generation). Plastics are a rapidly growing segment of MSW and are found in a wide variety of products, such as durable and non-durable goods, containers, packaging, furniture, etc. The resins used for these applications include high-density polyethylene (HDPE), low and linear-low density polyethylene (LDPE and LLDPE), polyethylene terephthalate (PET), polypropylene (PP), polystryene (PS), polyvinyl chloride (PVC), and others. Although most of the above resins are being recycled, the recovery level of HDPE and PET is substantially higher than the others.
Post-consumer polymeric waste, as opposed to industrial plastic waste, typically includes substantial quantities of plastic bottles, containers and packaging materials. Plastic bottles are molded of different polymeric materials depending upon the product they are to contain. For example, plastic bottles for water, milk, and household chemicals typically are made of high density polyethylene (HDPE), while soft drink bottles are typically made of polyethylene terephthalate (PET) with or without base caps made from high density polyethylene (HDPE). Generally, HDPE bottles account for approximately 50-60% and PET bottles account for approximately 20-30% of the bottles used by consumers. The balance of bottles, bottle caps and other containers used by consumers comprises other polymeric materials, such as low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and other resins and multi-layered materials.
Plastic packaging materials also are made of a wide variety of polymers. For example, according to Plastics Compounding, Nov/Dec, 1992, the following polymers were used in packaging material in the %Us set forth: 27% LDPE, 21% HDPE, 16% PS, 16% PP, and 5% PET. Film waste in the U.S. in 1996 contained 15% HDPE, 68% LDPE, 13% PP, 2% PS AND 2% PVC according to an EPA update. Such film waste is formed from bags, packaging and shrink wrap films.
Post-industrial plastic waste can comprise polyolefins, PS, PET and other polymeric materials used for plastic packaging.
Currently, collection of plastic waste material exceeds the market demand for recycled plastic products as a result of the dearth of viable recycling technologies that are low cost and produce high quality recycled plastic products. One recycling approach has involved the high energy consuming batch grinding of commingled, unsorted mixed color plastic waste to form flake scrap material, melt processing and pelletizing the melt processed material to pellets, and extruding the pelletized plastic waste to form recycled plastic products. However, recycled plastic products made in this manner suffer from severe deficiencies that render the products unsatisfactory for many purposes and are of inferior, low value compared to products made of virgin polymeric materials. For example, these recycled plastic products exhibit inferior mechanical properties (e.g. tensile, flexural and notched izod impact strength) and inferior appearance in terms of color (dark brown or gray color) with streaking of colors within the molded product as a result of the chemical incompatibility of the different polymers present in the initial plastic waste stream and variations in the plastic waste stream composition over time.
A typical example of a low value, recycled plastic product is recycled plastic lumber having a dark brown or gray color with noticeable color streaking and inferior mechanical properties compared to components molded of virgin materials. As a result of the less than pleasing appearance, recycled plastic lumber is oftentimes painted to improve its appeal to the customer, or expensive pigments and other additives are added to the feedstock during the manufacturing process to this end. However, the cost of the recycled product is increased thereby.
Furthermore, certain melt processing techniques, such as blow molding, rotational molding, extrusion (e.g. extruded PVC pipe and profiles), and spray coating, require a plastic powder feedstock. That is, the flake scrap material is not directly melt processable to articles of manufacture by such powder feedstock-requiring melt processing techniques. To be useful as feedstock in such melt processing techniques, sorted or unsorted flake scrap material produced by batch grinding must be pelletized and then ground to powder form. The need to pelletize and grind sorted or unsorted flake scrap polymeric material prior to such melt processing adds considerably to the cost and complexity of recycling scrap plastics as well as the capital equipment expenditures required.
Currently used injection molding techniques require plastic pellets for high speed production of molded parts. Although unsorted, commingled flake scrap materials could be pelletized to provide feedstock for injection molding, the resultant molded products would suffer from the types of deficiencies discussed above attributable to polymer incompatibility.
So-called compatibilizing agents and/or reinforcing agents can be added to flake plastic scrap material comprising chemically incompatible polymers in attempts to produce a recycled plastic product exhibiting more desirable characteristics. However, addition of these agents to the plastic scrap material makes recycling more difficult and adds considerably to its cost. The Mavel et al. U.S. Pat. No. 4,250,222 relates to this type of recycling approach and is representative of the disadvantages associated with such an approach to plastic recycling. In general, while there are available compatibilizing agents capable of providing compatibilization of binary polymeric blends, such materials are specific for the blend desired and costly to make and use. Acceptable compatibilizers for polymeric blends of three or more components simply do not exist.
Attempts have been made to sort commingled, post-consumer plastic scrap to overcome the polymer incompatibility problems associated with the recycling of commingled plastic scrap. To-date, HDPE and PET are recovered from plastic waste streams by recycling technologies requiring sorting of the commingled plastic materials. Sorting can require use of costly techniques, such as video cameras, electronic devices, infrared detectors, and organic xe2x80x9cmarkersxe2x80x9d, to provide effective segregation of like plastics. However, even sorted plastic waste can present problems in processing as a result of density and chemical differences among polymers falling in the same general class and made by different plastics manufacturers.
Further, sorted plastic scrap must be subjected to batch grinding to produce flake scrap material that then must be pelletized and ground again to provide powder feedstock for blow molding, rotational molding, some extruding, spray coating and other melt processing techniques that require powder feedstock.
The high cost of sorting has greatly limited widespread use of recycling approaches that require a sorting step. In particular, collected and sorted post-consumer plastic materials are usually more expensive than the corresponding virgin plastic materials. Thus, users of plastic materials are discouraged from using sorted, recycled plastic materials.
The plastics industry has dedicated significant technical and financial resources to increase plastics recycling from MSW. However, the properties of the discarded plastics are widely varied due to numerous suppliers, each of which use proprietary additive packages, fillers, and reinforcing agents. It has been established that it is not possible to control the consistency of the discarded feedstocks prior to recycling. Since mixed (commingled) plastics are incompatible with one another, their re-processing presents numerous challenges, including, but not limited to, a phase separation in the melt, delamination of molded parts, and inconsistent color, among others. Again, providing a compatibilizing agent to allow uniform blending of such diverse materials presents an insurmountable task, and, as of yet, such agents have not been made available.
In addition, the processing temperature of the plastics from MSW may differ by more than 100xc2x0 C., which would lead to a degradation of temperature-sensitive polymers. The most well-known example is of mixtures containing PET and PVC where PET requires high processing temperature above 270xc2x0C.; however, at this temperature, PVC would degrade. If the same mixture is processed at 170xc2x0 C., which is suitable for PVC, it would leave PET unmelted.
Because there is a limited use for mixed plastics waste, most of the discarded plastics are separated prior to reuse. The most common automated separation technologies are based on density difference, although manual sorting is still commonly used. The physical characteristics of the recycled material are sensitive to a change in composition and a presence of contaminants, such as labels, glue, adhesives, product residue, and dirt. For most applications of recycled plastics, a washing step would be an essential part of the reclamation process. After washing, single polymer streams are re-melted and re-pelletized for further re-use. Again, the sorting and washing procedures add costs which discourage wide spread recycling of plastics.
A new technology called Solid-State Shear Pulverization (S3P), developed by the Polymer Technology Center at Northwestern University, converts multicolor, mixed (commingled) plastics into a homogenous, pastel color powder, which is melt processable by all existing plastics fabrication techniques. U.S. Pat. No. 5,814,673 issued to Khait describes the S3P process. The entire content of this mentioned patent is herein incorporated by reference. The patented solid state shear pulverization process is disclosed as useful for pulverizing a commingled, multi-component polymeric scrap. That this process is capable of providing polymeric particles having good physical properties, uniform color and compatibilization without the addition of a compatibilization agent regardless of the differences in the chemical and thermodynamic nature of the polymeric scrap, and all done in the solid state, represents a drastic leap forward in processing multi-component polymeric scrap for recycle as well as providing a unique method of blending scrap and/or virgin polymers. Heretofore blending of even two polymeric materials has required the addition of expensive, specifically prepared compatibilizing agents, property-improving additives or simply could not have been accomplished by previous melt-blending techniques. U.S. Pat. No. 5,814,673 includes numerous examples directed to solid state shear pulverization of multi-component plastic materials including high density polyethylene, low density polyethylene, polypropylene, polyethylene terephthalate, polystyrene and polyvinyl chloride. In all but two of the three or more component blends, H-PE was the majority component. In the other two blends, PET was the major component. In none of the examples described, is there a multi-component blend which is formed by the solid state shear pulverization process which yields a notched izod impact strength of greater than 1.0 ft.-lb./in. Thus, while improvement in compatibility between diverse polymers is certainly disclosed as well as forming a uniform colored polymeric powder from multi-component and multicolored polymer blends and further describing that the S3P blends have improved tensile strength, the patent does not otherwise disclose the blends which form the basis of this invention. One of such blends formed from film waste by the S3P process has surprisingly improved notched izod impact strength and elongation.
It is therefore an object of the invention to provide a method of processing multi-component polymeric materials, such as sorted or unsorted, commingled scrap polymeric material, by solid state pulverization to produce pulverized particulates (e.g. powder) that can be directly formed to shape by powder feedstock-using conventional melt processing techniques.
It is another object of the invention to provide a method of processing multi-component polymeric materials, such as sorted or unsorted, commingled scrap polymeric materials, having mixed colors by solid state pulverization to produce pulverized particulates that are melt processable to a substantially homogeneous light color without color streaking or marbleizing despite being produced from the mixed color scrap materials.
It is a further object of the present invention to provide a method of recycling commingled scrap multi-component polymeric materials without sortation and in a manner to achieve in-situ compatibilization of different polymers present and produce recycled polymeric particulates without the need for a compatibilizing agent.
It is still another object of the present invention to provide solid state pulverized polymeric particulates that are suitable as powder feedstock for melt processing by blow molding, rotational molding, some extruding, spray coating and other powder feedstock-using melt processing techniques.
It is still a further object of the invention to produce articles of manufacture, including molded parts and coatings, made from the aforementioned solid state pulverized polymeric particulates.
It is still yet another object of the invention to provide a method of processing multi-component polymeric materials in a manner to achieve in-situ compatibilization of a mixture of two or more incompatible polymers and provide homogenous polymer blends having high notched izod impact strength and/or elongation.
The present invention provides in one aspect a method of making polymeric particulates (e.g. powder) wherein sorted or unsorted, commingled multi-component polymeric scrap material, is supplied to pulverizer screw means rotated to transport the material along the length thereof and in the solid state convert the material to pulverized particulates (e.g. powder) that are melt processable directly by conventional blow molding, rotational molding, extrusion, spray coating and other melt processing techniques requiring a powder feedstock. This avoids the need for and costs associated with flake pelletizing and pellet grinding operations heretofore required.
The solid state pulverized particulates also are melt processable by conventional molding, extruding, spray coating and the like to form articles of manufacture having a substantially homogenous color appearance without color streaking or marbleizing. This color homogeneity is achievable regardless of whether the particulates include mixed color polymeric material of the same or different composition. This avoids the need for the addition of pigments and/or compatibilizing agents to the feedstock and the need to paint the molded or extruded product to hide unpleasing colors and color streaking.
The present invention provides in another aspect a method of making polymeric particulates wherein polymeric material, such as unsorted multi-component polymeric scrap material, comprising two or more thermodynamically incompatible polymers is supplied to pulverizer screw means rotated to transport the material along the length thereof and subject the material to solid state pulverization and in-situ polymer compatibilization. In-situ polymer compatibilization is evidenced, in one instance, by the resulting pulverized polymeric particulates exhibiting a thermogram different from that of the precursor unpulverized material. For example, the pulverized particulates of the invention exhibit a melting peak and/or crystallization peak quite different from those of the unpulverized material. Moreover, molded articles produced from the pulverized particulates of the invention exhibit increased tensile strengths and lack of delamination upon breaking in mechanical testing, this being a further indication of in-situ polymer compatibilization.
In an important aspect of this invention, multicomponent plastic scrap of a configuration equivalent to that found in municipal solid waste is converted to polymeric particulates by solid state shear pulverization. This scrap has as the major component LDPE. A polymeric particulate formed from a waste stream of multicomponent polymeric film which has undergone solid state shear pulverization can be molded into articles which exhibit extraordinary notched izod impact strength, including notched izod strengths of at least 5.0 ft.-lb./in. and/or exceptional elongation.
In practicing the present invention, the multi-component polymeric scrap material can include thermoplastics, polymer blends, polymer alloys, thermosets, elastomers and other polymeric materials. Typically, the polymeric material is comminuted to flake form by grinding, chopping or shredding using conventional equipment prior to pulverization. The pulverization process uses as scrap feedstock a material that is in a physical form (e.g. comminuted flakes) commonly available from scrap collections and municipal recycling centers.
The polymeric material is only subjected to limited frictional heating generated during the initial stage of the pulverization operation by engagement with the rotating screws. That is, solid state shear pulverization of the polymeric material preferably is conducted without heating of the material by any external pulverizer barrel heating device. Temperature control of the polymeric material during the pulverization operation is thereby facilitated to reduce degradation of the polymers and dye materials used with the feedstock polymers. Energy consumption during the pulverization operation also is reduced.
The present invention provides in still another aspect a method of making an article of manufacture having a substantially homogenous color from mixed-color polymeric material, such as sorted or unsorted, commingled polymeric scrap material. In this embodiment of the invention, mixed-color polymeric material of the same or different composition is supplied to pulverizer screw means rotated to transport the polymeric material along the length thereof to subject the material to solid state pulverization to form pulverized particulates. The pulverized particulates are molded, extruded or otherwise melt processed to form a substantially homogeneously colored shape characterized by the absence of color streaking and marbleizing, despite the particulates originating from mixed-color polymeric material. Typically, the pulverized powder is processable to a substantially homogenous pastel color tone corresponding to a dominant color of a particular scrap component in the feedstock.
The present invention also provides solid state pulverized particulates produced from scrap polymeric material wherein the particulates are suitable as powder feedstock, without conventional melt pelletizing and pellet grinding, for direct melt processing to shape using blow molding, rotational molding, some extrusion, spray coating, and other powder feedstock-using techniques.
The present invention further provides solid state pulverized polymeric particulates comprising three or more otherwise thermodynamically incompatible polymers produced from commingled, unsorted polymeric scrap materials. The polymers are in-situ compatibilized by solid state shear pulverization as evidenced by one or more different thermogram characteristics between recycled particulates of the invention and unpulverized polymeric material. Typically, the solid state pulverized particulates exhibit enhanced reactivity as compared to the unpulverized polymeric material.
Moreover, the present invention provides solid state pulverized polymeric particulates that exhibit, pulverized and as-melt processed, a substantially homogenous color despite being pulverized from mixed-color scrap material.
Articles of manufacture and powder coatings produced from the solid state pulverized particulates of the present invention exhibit mechanical properties generally superior to those exhibited by like processed flake polymeric material of the same composition depending on the polymer components involved. Notched izod impact strength substantially greater than even that expected from a mixture of compatible polymers coupled with excellent elongation and melt flow properties are achieved. Importantly, they also exhibit a substantially homogeneous color characterized by the absence of color streaking or marbleizing.
The present invention is advantageous in that the pulverized particulates are suitable for direct use as powder feedstock for powder feedstock-using conventional melt processing techniques by elimination of pelletizing and consequent pellet grinding operations. Moreover, commingled scrap polymer materials, virgin polymeric materials and mixtures thereof can be processed in a manner to achieve in-situ compatibilization of different polymers in a once-through pulverization operation without the need for a compatibilizing agent and without sortation in the case of commingled scrap feedstock. The pulverized particulates may be mixed with fillers, reinforcing agents, flame retardants, antioxidants and other additives commonly used in the plastics industry if desired.
The present invention can provide a high value, low cost recycled particulates product, as well as products molded or otherwise melt processed therefrom, thereby increasing utilization of available plastic scrap.
The aforementioned objects and advantages will become more readily apparent from the following detailed description and drawings.